Method and Apparatus for Determining a Measure of a Thickness of a Polishing Pad of a Polishing Machine

ABSTRACT

An apparatus for determining a measure of a thickness of a polishing pad of a polishing machine includes a detector and a determiner. The detector is configured to detect a position of a carrier of an element to be polished in a pressing direction while the element is pressed by the carrier in the pressing direction against the polishing pad with a defined pressure. The detector is further configured to output a signal indicative of the position of the carrier. The determiner is configured to determine the measure of the thickness of the polishing pad based on the signal indicative of the position of the carrier.

TECHNICAL FIELD

Embodiments of the present invention refer to a method and an apparatus for determining a measure of a thickness of a polishing pad of a polishing machine and to a polishing machine.

BACKGROUND

A polishing machine is used to polish an element, e.g., a wafer, in order to provide a plan surface of the element. For this purpose, the surface of the element is abraded and leveled by using a polishing pad. In the process of polishing the element, the element is moved and/or rotated relatively in parallel with the polishing pad while the element is pressed by a carrier of the polishing machine against the polishing pad. The polishing process, or in more detail, several polishing processes, cause an abrasion of the polishing pad which represents a wear and tear part of the polishing machine. Therefore, a polishing pad is replaced when a certain number of elements have been polished or when a minimum residual thickness of the polishing pad is reached. Due to the small pad thickness, e.g., 1200 μm, it is a difficult task to determine and monitor the thickness of the polishing pad exactly.

SUMMARY OF THE INVENTION

An embodiment provides an apparatus for determining a measure of a thickness of a polishing pad of a polishing machine. The apparatus comprises a detector configured to detect a position of a carrier of an element to be polished in a pressing direction while the element is pressed by the carrier in the pressing direction against the polishing pad with a defined pressure and to output a signal indicative of the position of the carrier. The apparatus further comprises a determiner configured to determine the measure of the thickness of the polishing pad based on the signal indicative of the position of the carrier.

A further embodiment provides an apparatus for determining a measure of a thickness of a polishing pad of a polishing machine. The apparatus comprises a detector configured to detect a position of a carrier of an element to be polished in a polishing direction while the element is pressed by the carrier in the pressing direction against the polishing pad with a defined pressure, wherein the detector is configured to output a signal indicative of the position of the carrier. The apparatus further comprises a determiner configured to determine the measure of the thickness of the polishing pad based on the signal indicative of the position of the carrier and on the basis of calibration data obtained in advance using the polishing pad of a known thickness, a predetermined pressure and a predetermined thickness of the element, wherein the determiner is configured to output an alarm-signal if the determined thickness of the polishing pad falls below a threshold value.

Another embodiment provides a polishing machine for polishing an element. The polishing machine comprises a platen on which a polishing pad is fixed and a carrier of the element to be polished. The carrier is configured to move in a pressing direction and to press the element in the pressing direction against the polishing pad with a defined pressure and to move and/or rotate the element relatively in parallel with a polishing pad. The polishing machine comprises a detector configured to detect a position of the carrier while the element is pressed by the carrier in the pressing direction against the polishing pad with the defined pressure and to output a signal indicative of the position of the carrier. The polishing machine further comprises a determiner configured to determine the measure of the thickness of the polishing pad based on the signal indicative of a position of a carrier.

A further embodiment provides a method for determining a measure of a thickness of a polishing pad of a polishing machine. The method comprises: detecting a position of a carrier of an element to be polished in a pressing direction while the element is pressed by the carrier in the pressing direction against the polishing pad with a defined pressure; outputting a signal indicative of the position of the carrier. The measure of the thickness of the polishing pad is determined based on the signal indicative of the position of a carrier.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments according to the present invention will subsequently be discussed making reference to the enclosed figures in which:

FIG. 1 shows a schematic block diagram of a detector and a determiner applied to a polishing machine according to an embodiment;

FIG. 2 a shows a schematic diagram of a pressure exerted by a carrier and a consequent compression of a polishing pad to illustrate the influence of the pressure on a determination of a measure of the thickness of the polishing pad;

FIG. 2 b shows a schematic view of a polishing machine having five carriers to illustrate the influence of a number of carriers used on a determination of a measure of the thickness of a polishing pad;

FIG. 2 c shows a schematic multidimensional table to illustrate a relation of a measure of the thickness with a signal indicative of a position of a carrier depending on factors of influence;

FIG. 3 shows a diagram of a plurality of signals indicative of the position of a carrier consecutively in time plotted over the time during a polishing process;

FIG. 4 shows a schematic diagram of determined measures of thickness of seven polishing pads plotted over the time;

FIG. 5 a shows a schematic view of an embodiment having an optical detector and a determiner applied to a polishing machine; and

FIG. 5 b shows a schematic diagram of a signal indicative of a position of a carrier detected by the optical detector according to the embodiment of FIG. 5 a.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

FIG. 1 shows an embodiment of an apparatus 10 for determining a measure of the thickness t of a polishing pad 12 of a polishing machine 14. The apparatus 10 comprises a detector 16 and a determiner 18. The detector 16 is configured to detect a position 20 of a carrier 22 of the polishing machine 14 in a pressing direction 24 and to output a signal 26 indicative of the position 20 of the carrier 22. The apparatus 10 is, in this embodiment, applied to the polishing machine 14 comprising a platen 28 on which the polishing pad 12 is fixed. The polishing machine 14 further comprises the carrier 22 of an element 30 to be polished which is pressed by the carrier 22 in the pressing direction 24 against the polishing pad 12 with a defined pressure p₁, e.g., 1600 N. The carrier 22 which holds the element 30 is moveable relative to the platen 28 in the pressing direction 24, perpendicular to the platen 28, and thus may have different positions 20 in the pressing direction 24.

In the following, the function of the apparatus 10 for determining the measure of the thickness t based on the position 20 of the carrier 22 will be described. The thickness t of the polishing pad 12 is the extension thereof in the pressing direction 24.

The position 20 of the carrier 22 in the pressing direction 24 depends on the thickness t of the polishing pad 12 while the element 30 is pressed by the carrier 22 with the defined pressure p₁. The detector 16 detects the position 20 and outputs the signal 26 indicative of the position 20, for example, a voltage, to the determiner 18. The determiner 18 determines on the basis of the signal 26 the measure of the thickness t on an assumption of a known or constant thickness of the element 30. The measure may be an absolute or a relative value of the thickness t. It is beneficial that the measure of the thickness t of the polishing pad 12 and thus an abrasion of the polishing pad 12 may be detected and/or monitored during the polishing process of the element 30. So the polishing pad 12 may be used to full capacity or, in other words, up to the minimum predefined residual thickness t. In addition, the apparatus 10 may be applied to different kinds of existing polishing machines.

Embodiments are based on the recognition that it is possible to determine a measure of the thickness t of a polishing pad indirectly by detecting the position of a carrier pressing an element to be polished against the polishing pad. The position of the carrier can be detected in an easy manner using appropriate detectors, such as a laser based optical sensor or a capacitive transducer. Accordingly, the thickness t of the polishing pad can be determined in a reliable manner even during a polishing process without having to measure the thickness t of the polishing pad directly.

In embodiments, conditions while determining the measure of the thickness t of the polishing pad are controlled to correspond to conditions while obtaining calibration data so that the measure of the thickness t of the polishing pad can be derived from the output signal of the detector directly by accessing the calibration data using the output signal of the detector.

In other embodiments, calibration data are obtained for different conditions, i.e., factors of influence, such as different pressures, a different number of elements pressed against the polishing pad and/or different thicknesses of the element to be polished. In such embodiments, lookup tables may be created based on the calibration data and are accessed based on one or more of the actual conditions present when determining the thickness t of the polishing pad. Appropriate sensors for detecting the actual conditions may be provided. In other embodiments, the actual conditions may be input to the device by an operator. Accessing the lookup table to take the actual conditions into account may be regarded as taking one or more correction factors into consideration in determining the measure of the thickness t of the polishing pad.

With respect to FIG. 2 a to FIG. 2 c, factors of influence on the determination of the measure of the thickness t of the polishing pad will be described. The factors of influence may be the pressure p with which the element is pressed against the polishing pad, an actual thickness of the element to be polished, a characteristic of the carrier used, e.g., a thickness of the carrier, and a number of elements pressed to the polishing pad by respective carriers in parallel.

FIG. 2 a shows exemplarily a compression of the polishing pad dependent on the pressure p with which the element is pressed against the polishing pad by the carrier of a polishing machine. The diagram shows a graph 32 of measured points of the polishing pad compression plotted over the pressure p. The graph 32 obtained by an experiment shows a substantially linear dependence on the two measurement parameters on a coefficient of determination of 99.3%, as illustrated by a linear graph 34. The following specification proceeds on the assumption of a linear dependence on the pressure p and the position of the carrier on the basis of which the thickness t of the polishing pad is determined. The compression of the polishing pad caused by the pressure p results from an elastic deformation of the polishing pad and has no impact on the thickness t, but on its determination.

The dependence on the pressure p influences the determination of the measure of the thickness t. Thus, according to embodiments, the defined pressure p₁ is controlled to correspond to a predetermined pressure p₂ with which the element was pressed against the polishing pad when obtaining calibration data. Thus, the detector detects the position of the carrier when conditions in terms of pressure p correspond to conditions during obtaining calibration data, i.e., when the defined pressure p₁ is equal to the predetermined pressure p₂. In other embodiments, the determiner may use a correction factor reflecting a difference between the defined pressure p₁ and the predetermined pressure p₂ in determining the thickness of the polishing pad.

The actual thickness of the element to be polished or being polished is a further factor which influences the determination of the measure of the thickness t directly. According to embodiments, the actual thickness of the element corresponds to a predetermined thickness of an element used while obtaining the calibration data. In other embodiments, the determiner may when determining the thickness t of the polishing pad use a correction factor reflecting a difference between the actual thickness of the element at the time the thickness t of the polishing pad is determined and the predetermined thickness.

FIG. 2 b shows schematically a polishing machine 40 having five carriers for five elements to be polished. The polishing machine 40 substantially corresponds to the polishing machine 14 shown with respect to FIG. 1, but, in contrast, the polishing machine 40 has five moveable carriers 42 a, 42 b, 42 c, 42 d and 42 e. Each carrier 42 a, 42 b, 42 c, 42 d, and 42 e comprises a respective down force cylinder 44 a, 44 b, 44 c, 44 d and 44 e configured to press an element 46 a, 46 b, 46 c, 46 d and 46 e against the polishing pad 12 in the pressing direction 24 with the defined pressure p₁. The detector 16 is associated with the carrier 42 b in order to detect the position 20 of the carrier 42 b, as described above.

The position 20 of the carrier 42 b detected by the detector 16 depends on a number and/or a position of carriers 42 a, 42 b, 42 c, 42 d and 42 e used in parallel or, in other words, on a number and/or a position of elements pressed in parallel against the polishing pad 12. Background thereof is that a load distribution on the polishing pad 12 varies dependent on a respective carrier configuration, i.e., dependent on the number of carriers actually in use or dependent on the position of carriers actually in use. The load distribution differs, for example, if carriers having a small distance to each other (e.g., carrier 42 b and 42 c) or carriers having a larger distance to each other (e.g., carrier 42 b and 42 d) are used. In embodiments, the detector 16 detects the position 20 of the carrier 42 b while the conditions in terms of the number and/or the position of carriers 42 a, 42 b, 42 c, 42 d and 42 e used and the number and/or the position of elements 46 a, 46 b, 46 c, 46 d and 46 e to be polished correspond to the conditions during obtaining the calibration data. In other embodiments, the determiner may when determining the thickness t of the polishing pad use a correction factor reflecting a difference between the number and/or the position of carriers (or elements) actually in use and the number and/or the position of carriers (or elements) used during obtaining the calibration data.

Furthermore, the carrier itself has an influence on the determination of the measure of the thickness t of the polishing pad due to, for example, a thickness of the carrier or its geometric tolerances. Thus, the calibration data may be obtained after each change of the carrier in order to ensure comparable conditions in terms of the carrier during the determination of the thickness t and the obtaining of the calibration data.

FIG. 2 c shows a multidimensional table of the thickness t dependent on the signal indicative of the position of the carrier and dependent on above discussed factors of influence, such as the defined pressure p₁ and the numbers of elements (see 46 a, 46 b, 46 c, 46 d and 46 e) pressed to the polishing pad in parallel. In an area 47 of the table, the signals indicative of the position (see column 48 a) are assigned to respective values of the thickness t (see column 49 a). In each row, a signal, e.g., signal 48 a_3, complies with a respective absolute value of the thickness t, e.g., 49 a_3, for the case of using one carrier. In a second dimension, the signals (see column 48 a) are associated to the respective values of the thickness t (see column 49 a, 49 b, 49 c, 49 d and 49 e) dependent on the number of carriers used. In a third dimension, the respective signals (cf. column 48 a, 48 b, 48 c, 48 d and 48 e) are assigned to the defined pressure p₁ with which the element is pressed against the polishing pad during the determination of the measure of the thickness t. The defined pressure p₁ is described by a value depending on the predetermined pressure p₂. The multidimensional table may have further dimensions such as for correcting an influence of the actual thickness of the element which may differ from the predefined thickness.

Obtaining the values of a table assigning the values of the thickness t to the output signals of the detector, the respective pressures p, the number of carriers used and/or the thickness of the element may be part of the obtaining of the calibration data using a polishing pad of a known thickness t. In this embodiment, the output signal of the detector, the values of the thickness t and the pressure p have a linear dependence, as described above. The dependence between the output signal of the detector, the value of the thickness t, the number of carriers and/or the pressure p may be linear or non-linear.

The described assignment using the calibration table corresponds to the determination of the thickness t by applying one or more correction factors, wherein a first correction factor depends on a difference between the defined pressure p₁ and the predetermined pressure p₂, a second correction factor depends on a difference between the actual thickness of the element to be polished and the predetermined thickness and a third correction factor depends on a number of elements pressed to the polishing pad by the respective carriers (see 42 a, 42 b, 42 c, 42 d and 42 d) in parallel. According to an embodiment, the determiner is configured to determine the measure of the thickness t by using such a multidimensional lookup table. It is beneficial that the thickness t of the polishing pad may be determined under different conditions, e.g., a different defined pressure p₁.

In other embodiments, the apparatus is controlled so that the conditions correspond to all conditions of obtaining the calibration data so that the thickness t can be determined from the output signal of the detector directly such as by using an assignment table having a single column only. In other embodiments, one or more correction factors are used dependent on which conditions do not correspond to the conditions of the calibration.

FIG. 3 shows a diagram of the signal indicative of the position of the carrier plotted over the time of a polishing process. Here, a graph 50 shows a plurality of signals detected consecutively in time during three phases of the polishing process. The first phase 52 is from the beginning of the polishing process to the point of time when the polishing machine is steady-state. The second phase 54 represents the main interval of the polishing processing in which the polishing machine is steady-state. The third phase 56 represents the interval just before finishing the polishing process and lifting the carrier.

Graph 50 shows in the first phase 52 and the third phase 56 high and distorted values due to the lowering and lifting of the carrier. In the second phase 54 the signal shows a constant curve shape with small oscillations. The oscillations of the signal may be caused by oscillations of the polishing machine during the polishing process. Due to the constant values in the second interval 54, the detector according to this embodiment detects the position of the carrier at a predetermined time of the polishing process within this interval 54. In order to eliminate the oscillations of the signal, the determiner, according to embodiments, determines the measure of the thickness t based on an average of a plurality of signals consecutively in time during a further interval 58 which is a proper subset of the interval 54 and may have duration of 30 seconds.

The abrasion of the element being polished during the interval 58 or during a singular polishing process is comparably small so that the abrasion does not influence the determination of the measure of the thickness t significantly. Furthermore, the detector may detect the position at the predetermined time of the polishing process so that determinations of a first and second measure of the thickness t during a first and a second polishing process are comparable.

FIG. 4 shows a diagram of different measurements of the thickness t determined by the apparatus and method described above. The diagram is plotted over the time of approximately seven weeks and shows measuring points of seven different polishing pads 61 a, 61 b, 61 c, 61 d, 61 e, 61 f and 61 g. Each polishing pad has an initial thickness t of approximately 1200 μm and is used up to the allowed residual thickness t of the polishing pad, e.g., 800 μm, or even longer.

The value of the allowed residual polishing pad thickness t represents a threshold value 60. In order to use a polishing pad to full capacity (up to the minimum predefined residual thickness t), the determiner may be configured to output an alarm-signal if the determined thickness t of the polishing pad falls below the threshold value 60. The threshold value 60 may be determined on an individual basis for each kind of polishing pads. The initial thickness t of the polishing pad may be subject to variations in a limited range of approximately 80 μm. Therefore, the determiner may be calibrated to a known initial thickness t of the polishing pad after changing the polishing pad in order to set the threshold value 60 relative to the initial thickness t of the polishing pad. This enables to set the alarm-signal based on an abrasion of the polishing pad.

In the following, a preferred embodiment having an optical detector will be discussed with respect to FIGS. 5 a and 5 b.

FIG. 5 a shows an embodiment comparable to the embodiment of FIG. 1 a, wherein the detector is formed by an optical detector 62. The detector 62 comprises a reflector 64 associated with a carrier 22 such that the position 20 of the carrier 22 is coupled with a position 65 of the reflector 64. The detector 62 further comprises a fixed signal source 66, for example, a laser, and a fixed sensor 68, e.g. a CCD-chip. The reflector 64 may be displaced by 30 mm (illustrated by arrow d) relative to the signal source 66 and may have a range of motion of +/−5 mm, illustrated by two positions 65 b and 65 c. In this embodiment, the signal source 66 is arranged such that an electromagnetic wave 70 is emitted along the pressing direction 24 to the reflector 64. The sensor 68 is angled relative to the signal source 66 such that it is configured to receive an electromagnetic wave 71 a, 71 b or 71 c diffusely reflected by the reflector 64. The signal source 66 may further comprise a lens 72 via which the electromagnetic wave 70 is emitted. The sensor 68 may further comprise a lens 74 via which the electromagnetic wave 71 a, 71 b or 71 c is received.

In the following the function of the detector 62 of this embodiment will be discussed.

The signal source 66 emits the electromagnetic wave 70 which is reflected by the reflector 64 to the sensor 68. The sensor 68 receives the electromagnetic wave 71 a, 71 b or 71 c under an angle of incidence α which depends on the position 65 of the reflector 64. For example, if the reflector 64 is in a position 65 a it reflects the electromagnetic wave 70 such that the sensor 68 receives the electromagnetic wave 71 a under a first angle of incidence α. Analogously the electromagnetic wave 71 b or 71 c is received under a second or third angle of incidence α if the reflector 64 is in position 65 b and 65 c, respectively. The sensor 68 is configured to obtain the angle of incidence α. In the embodiment, the obtained angle of incidence α corresponds to the electrical signal 26 which is output by the detector 62 or, in more detail, by the sensor 68 to the determiner 18. As described with respect to FIG. 1, the determiner determines the thickness t of the polishing pad 12 on the basis of the signal 26 indicative of the angle of incidence α and thus indicative of the position 65 of the reflector 64 and the position 20 of the carrier 22, respectively.

Below, the detection of the angle of incidence α will be described. FIG. 5 b schematically shows an intensity spectrum plotted over a position parameter x (width) of the sensor 68. An exemplary graph 78 of the received electromagnetic wave 71 b shows a maximum intensity at a position 79 b.

The diffusely reflected electromagnetic wave 71 a, 71 b or 71 c is projected by the lens 74 to the sensor 68 such that the position x of the maximum intensity depends on the angle of incidence α. Exemplarily, three different positions x 79 a, 79 b and 79 c for the respective first, second and third angle of incidence α (of the respective electromagnetic wave 71 a, 71 b and 71 c) are illustrated. It is beneficial that the angle of incidence α and thus the thickness t of the polishing pad 12 may be obtained exactly due to the distinct maximum intensity of the graph 78.

An alternative to the determination of the thickness t of the polishing pad on the basis of the angle of incidence is to determine the thickness t by using a different optical sensor. An embodiment for such an optical sensor would be to obtain the travel time of the electromagnetic wave 70 emitted by the fixed signal source 66 reflected by the reflector 64 directly to a fixed sensor. The fixed sensor is configured to obtain the travel time which depends on the position of the reflector or, in more detail, on the distance d between the signal source 66 and the reflector 64 and the distance between the sensor and the reflector 64 as well as on the velocity of light. Therefore, the determiner 18 determines the position of the carrier 22 on the basis of a time difference between the point of time of emitting the electromagnetic wave 70 and the point of time of receiving the reflected electromagnetic wave 71 a, 71 b or 71 c.

Detectors of alternative embodiments may use an electrical sensor such as a Hall Effect sensor, a potentiometer or a capacitive transducer.

Embodiments relate to the polishing machine 14 which comprises the platen 28 on which the polishing pad 12 is fixed, and the moveable carrier 22 of the element 30 to be polished. The polishing machine 14 further comprises the detector 16 configured to detect the position 20 of the carrier 22 while the element 30 is pressed by the carrier 22 against the polishing pad 12 and the determiner 18 which is configured to determine the measure of the thickness t of the polishing pad 12 (see FIG. 1). The carrier 22 is configured to rotate and/or move in parallel relatively to the platen 28 in order to polish the element 30. Optionally, the platen 28 may rotate. The polishing machine 14 may be configured to press a plurality of elements against the polishing pad 12 or may comprise a plurality of carriers each configured to press an element against the polishing pad 12, as described with respect to FIG. 2 b. Furthermore, the polishing machine may comprise one or more down force cylinders (see 44 a, 44 b, 44 c, 44 d and 44 e) for pressing the one or more carriers against the polishing pad 12. In this case the detector 16 of the polishing machine 14 may be configured to detect the position of the down force cylinder for determining the thickness t of the polishing pad 12.

The polishing machine 14 may be controlled by a controller which is configured to provide values such as the defined pressure p₁, the thickness of the element and the number of elements pressed to the polishing pad. In order to determine the defined pressure p₁ the polishing machine 14 may comprise a pressure sensor and output the obtained defined pressure p₁ to the determiner. The values provided by the controller and/or output by the pressure sensor may be used by the determiner 18 for the determination of the thickness t, such as for accessing a calibration table.

In alternative embodiments, at least two polishing pads may be fixed on the platen 28 such that the measure of the thickness t determined by the apparatus, e.g., the apparatus 10, corresponds to a measure of a thickness of at least two polishing pads.

In alternative embodiments, the determination of the measure of the thickness t may be performed on the basis of a plurality of signals indicative of positions of a plurality of carriers. The signals are output by a plurality of detectors each configured to detect the position of the respective carrier.

Although some aspects have been described in the context of an apparatus, it is clear that these aspects also represent a description of the corresponding method for determining the measure of the thickness t of the polishing pad, where a block or device corresponds to a method step or a feature of a method step. Analogously, aspects described in the context of a method step also represent a description of a corresponding block or item or feature of a corresponding apparatus. Some or all of the method steps may be executed by (or using) a hardware apparatus, like, for example, a microprocessor, a programmable computer or an electronic circuit. In some embodiments, some one or more of the most important method steps may be executed by such an apparatus.

A further embodiment of the inventive methods is, therefore, a data carrier (or a digital storage medium, or a computer-readable medium) comprising, recorded thereon, the computer program for performing one of the methods described herein. The data carrier, the digital storage medium or the recorded medium are typically tangible and/or non-transitionary. A non-transitory computer readable medium comprises instructions, which, when executed by a processor of an apparatus for determining the measure of the thickness of the polishing pad, cause the apparatus to perform the above described method.

Depending on certain implementation requirements, embodiments of the invention can be implemented in hardware or in software. The implementation can be performed using a digital storage medium, for example, a Blu-Ray, a CD, an EPROM or a FLASH memory, having electronically readable control signals stored thereon, which cooperate (or are capable of cooperating) with a programmable computer system such that the respective method is performed. Therefore, the digital storage medium may be computer readable.

Other embodiments comprise a computer program for performing one of the methods described herein, stored on a machine readable carrier. In other words, an embodiment of the inventive method is, therefore, a computer program having a program code for performing one of the methods described herein, when the computer program runs on a computer.

A further embodiment comprises a processing means, for example, a computer, or a programmable logic device, configured to or adapted to perform one of the methods described herein. In some embodiments, a programmable logic device may be used to perform some or all of the functionalities of the methods described herein. In some embodiments, a field programmable gate array may cooperate with a microprocessor in order to perform one of the methods described herein. Generally, the methods are preferably performed by any hardware apparatus.

The above described embodiments are merely illustrative for the principles of the present invention. It is understood that modifications and variations of the arrangements and the details described herein will be apparent to others skilled in the art. It is the intent, therefore, to be limited only by the scope of the impending patent claims and not by the specific details presented by way of description and explanation of the embodiments herein. 

1. An apparatus for determining a measure of a thickness of a polishing pad of a polishing machine, the apparatus comprising: a detector configured to detect a position of a carrier of an element to be polished in a pressing direction while the element is pressed by the carrier in the pressing direction against the polishing pad with a defined pressure and to output a signal indicative of the position of the carrier; and a determiner configured to determine the measure of the thickness of the polishing pad based on the signal indicative of the position of the carrier.
 2. The apparatus of claim 1, wherein the determiner is configured to determine the measure of the thickness of the polishing pad by comparing the signal indicative of the position of the carrier to calibration data obtained in advance using the polishing pad of a known thickness, a predetermined pressure and a predetermined thickness of the element.
 3. The apparatus of claim 2, wherein the detector is configured to detect the position of the carrier while conditions in terms of at least one of the defined pressure, the carrier used and the element to be polished correspond to conditions during obtaining the calibration data.
 4. The apparatus of claim 2, wherein the determiner is configured to determine the measure of the thickness taking into consideration at least one of a first correction factor that depends on a difference between the defined pressure and the predetermined pressure, a second correction factor that depends on a difference between an actual thickness of an element to be polished and the predetermined thickness and a third correction factor that depends on a number and/or a position of elements pressed to the polishing pad by respective carriers in parallel.
 5. The apparatus of claim 1, wherein the detector is configured to detect the position of the carrier at a predetermined time after starting a process of polishing the element.
 6. The apparatus of claim 1, wherein the detector is configured to output a plurality of signals indicative of the position of the carrier consecutively in time, and wherein the determiner is configured to determine the measure of the thickness based on an average of the plurality of signals.
 7. The apparatus of claim 1, wherein the determiner is configured to output an alarm-signal if the determined thickness of the polishing pad falls below a threshold value.
 8. The apparatus of claim 1, wherein the detector comprises: a fixed signal source configured to emit an electromagnetic wave; a reflector associated with the carrier such that the position of the carrier is coupled with a position of the reflector; and a fixed sensor configured to obtain an angle of incidence of the electromagnetic wave reflected by the reflector; wherein the obtained angle of incidence depends on the position of the reflector.
 9. The apparatus of claim 8, wherein the signal source is a laser.
 10. The apparatus of claim 8, wherein the reflector reflects the electromagnetic wave diffusely; and wherein the sensor comprises a lens and an electronic light sensor and wherein the sensor is configured to obtain the angle of incidence of the reflected electromagnetic wave by obtaining a maximum intensity of the reflected electromagnetic wave, the maximum intensity is projected to the electronic light sensor by the lens.
 11. The apparatus of claim 1, wherein the detector comprises: a fixed signal source configured to emit an electromagnetic wave; a reflector associated with the carrier such that the position of the carrier is coupled with a position of the reflector; and a fixed sensor configured to obtain a travel time of the electromagnetic wave reflected by the reflector; wherein the obtained travel time depends on the position of the reflector.
 12. The apparatus of claim 1, wherein the detector comprises an electrical position sensor.
 13. An apparatus for determining a measure of a thickness of a polishing pad of a polishing machine, the apparatus comprising: a detector configured to detect a position of a carrier of an element to be polished in a pressing direction while the element is pressed by the carrier in the pressing direction against the polishing pad with a defined pressure, wherein the detector is configured to output a signal indicative of the position of the carrier; and a determiner configured to determine the measure of the thickness of the polishing pad based on the signal indicative of the position of the carrier and on the basis of calibration data obtained in advance using the polishing pad of a known thickness, a predetermined pressure and a predetermined thickness of the element, wherein the determiner is configured to output an alarm-signal if the determined thickness of the polishing pad falls below a threshold value.
 14. The apparatus of claim 13, wherein the determiner is configured to determine the measure of the thickness of the polishing pad when the defined pressure is equal the predetermined pressure and/or the defined thickness of the element is equal predetermined thickness of the element.
 15. A polishing machine for polishing an element, the polishing machine comprising: a platen on which a polishing pad is to be fixed; a carrier of the element to be polished, wherein the carrier is configured to move in a pressing direction so as to press the element in the pressing direction against the polishing pad with a defined pressure, wherein the polishing machine is configured to cause a relative movement or rotation between the element and the polishing pad in parallel with the polishing pad; a detector configured to detect a position of the carrier while the element is pressed by the carrier in the pressing direction against the polishing pad with the defined pressure and to output a signal indicative of the position of the carrier; and a determiner configured to determine a measure of the thickness of the polishing pad based on the signal indicative of the position of the carrier.
 16. The polishing machine of claim 15, wherein the carrier of the polishing machine is configured to press a plurality of elements against the polishing pad; or wherein the polishing machine comprises a plurality of carriers each configured to press an element to be polished against the polishing pad.
 17. The polishing machine of claim 15, further comprising a pressure sensor configured to determine the defined pressure.
 18. The polishing machine of claim 15, further comprising a controller configured to control the polishing machine and to provide at least one of a value of the defined pressure, a thickness of the element to be polished and a number and/or a position of elements pressed to the polishing pad by respective carriers in parallel; and wherein the determiner is configured to determine the measure of thickness of the polishing pad taking the at least one value in consideration.
 19. The polishing machine of claim 15, wherein the detector is an electrical or optical position sensor.
 20. The polishing machine of claim 15, further comprising one or more down force cylinders for pressing the one or more carrier against the polishing pad, wherein the detector is configured to detect a position of the down force cylinder.
 21. The polishing machine of claim 15, further comprising the polishing pad fixed on the platen.
 22. A method for determining a measure of a thickness of a polishing pad of a polishing machine, the method comprising: detecting a position of a carrier of an element to be polished in a pressing direction while the element is pressed by the carrier in the pressing direction against the polishing pad with a defined pressure; outputting a signal indicative of the position of the carrier; and determining the measure of the thickness of the polishing pad based on the signal indicative of the position of the carrier.
 23. The method of claim 22, further comprising obtaining calibration data in advance of using a polishing pad of known thickness and a thickness of the element; and wherein detecting the position of the carrier is performed while conditions in terms of at least one of the defined pressure, the carrier used and the element to be polished correspond to corresponding conditions during obtaining the calibration data.
 24. The method of claim 22, further comprising: obtaining calibration data in advance using a polishing pad of known thickness, a predetermined pressure and a predetermined thickness of the element; and correcting the signal indicative of the position based on at least one of a first correction factor that depends on a difference between the defined pressure and the predetermined pressure, a second correction factor that depends on a difference between an actual thickness of an element to be polished and the predetermined thickness and a third correction factor that depends on a number and/or a position of elements pressed to the polishing pad by respective carriers in parallel.
 25. The method of claim 22, wherein detecting the position of the carrier is performed at a predetermined time after starting a process of polishing the element.
 26. A non-transitory computer readable medium comprising instructions, which, when executed by a processor of an apparatus for determining a measure of a thickness of a polishing pad, causes the apparatus to perform a method comprising: detecting a position of a carrier of an element to be polished in a pressing direction while the element is pressed by the carrier in the pressing direction against the polishing pad with a defined pressure; outputting a signal indicative of the position of the carrier; and determining the measure of the thickness of the polishing pad based on the signal indicative of the position of the carrier. 